fix: coerce null _id to empty string in revoke fingerprint match (v0.1.7)

a key doc with an explicit `_id: null` made doc.get("_id", "") return None
instead of the default, so .startswith(prefix) raised an uncaught
AttributeError instead of the contracted clean [✘] + exit 1.

Signed-off-by: disqualifier <dev@disqualifier.me>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "envelope_authorizer"
-version = "0.1.6"
+version = "0.1.7"
 description = "CLI key-authorization manager for envelope_crypto"
 requires-python = ">=3.10"
 dependencies = [